The invention relates more particularly to a pneumatic brake booster for a motor vehicle, of the type which comprises a rigid casing inside which there can move a transverse partition delimiting in a sealed manner a front chamber, subjected to a first pressure, and a rear chamber, subjected to a second pressure that varies between the first pressure and a pressure higher-than the first pressure, which is able to act on an actuating rod of a master cylinder associated with the booster via a reaction disk, of the type which comprises a tubular moving piston which is slideably mounted in the casing and is secured to the moving partition, of the type which comprises a control rod moving in the piston selectively as a function of an axial input force directed forward against the action of a return force exerted on the rod by a return spring, of the type in which the movements of the control rod are able to determine the openings and closings of at least one axial “inlet” valve which is interposed between a pressure source subjected to the pressure higher than the first pressure and the rear chamber and of at least one axial “re-equalizing” valve that is axially offset with respect to the inlet valve, which is interposed between the front chamber and the rear chamber, to actuate the moving partition, and of the type in which a plunger, slideably mounted in the piston, passing through the moving partition and secured to the end of the control rod, is able to act directly upon the actuating rod of the master cylinder via the reaction disk, of the type which comprises:                a first transverse sealing element for the axial inlet valve, secured to the rear end of the plunger,        a first complementary transverse sealing element for the axial re-equalizing valve, consisting of at least one portion of the rear face of the moving partition.        
Numerous examples of boosters of the aforementioned type are known.
In such boosters, the first transverse sealing element for the axial inlet valve generally consists of a cup crimped on to a cylindrical surface of the rear end of the plunger. Fitting this cup presents major difficulties when the booster is being manufactured.
This is because this cup needs to be fixed to the plunger in a given axial position in order to guarantee satisfactory opening of the inlet valve.
Now, crimping operations generally lead to the introduction of axial clearances which are detrimental to the observance of strict axial positioning of said cup with respect to the plunger.